1. Field of the Invention
The present invention relates to a process of forging a steel, particularly steel articles having a complicated shape such as connecting rods and other load bearing parts used for the suspension assembly of automobiles and construction equipment.
2. Description of the Related Art
The conventional processes for producing machine parts by forging steel include hot forging, warm forging, and cold forging. Small articles having a simple shape are produced by cold forging and large articles having a complicated shape are produced by hot forging. Warm forging is particularly used for the high precision forming of stainless steel and other materials having a high resistance to deformation.
The recent trend of minimizing the weight of machine parts including those of automobiles necessitates steel materials with greater strength achieved by the addition of alloying elements in steel, resulting in an increased resistance to deformation which a forging tool cannot withstand. Moreover, a section modulus compensating for a reduction in stiffness due to weight reduction requires a complicated article shape causing a further reduction in the life of the forging tools used for forming thereof.
To solve this problem, it might be possible to reduce the resistance to deformation by using an elevated forging temperature higher than the conventional temperature of from 1000.degree. to 1250.degree. C., or it might be also possible to form a steel article directly from a molten state.
Forging a steel at such high temperatures, however, is not practically advantageous and is not actually done, because the elevated temperature causes an intense oxidation of steel during heating and forging thereof with a resulting degradation in product yield, article precision, and surface quality and because the formability of steel is not remarkably improved as expected due to a rapid drop of the material temperature when brought into contact with a forging die.
Such an elevated temperature forging is only reported on page 11 of "SEISAN-KENKYU (Study of Manufacture)", Feb. 1990, vol. 42, No. 2, page 11 published by the Institute of Industrial Science, University of Tokyo, in which a cast iron is heated to a half-molten state and forged. The half-molten state enables a material which is otherwise unforgeable to be forged without the occurrence of cracking. A cast iron can be brought into a half-molten state by heating to about 1000.degree. C., which is not higher than a normal temperature used in the forging of steels, and no particular measures are taken to control the heating condition and atmosphere for suppressing the oxidation and the working condition for improving the formability.
Steels have a melting point far higher than that of cast irons and are not forged at a temperature close to the melting point thereof because of the above-mentioned problems.
Cast irons are, of course, not applicable as a material for strength parts or load bearing parts necessary for automobiles, etc.
The "forging cast process" produces machine parts directly from a molten metal and is applied to the production of a suspension assembly of automobiles and other parts such as pistons as described in Kobe Steel Engineering Report, vol. 21, No. 3, page 57. Problems occur, however, in that the direct introduction of molten metal into a mold causes the molten metal to adhere to the mold wall thereby affecting the parting of products from the mold as well as the mold life.